1. Field of the Invention
The present invention relates to a voltage-dependent resistor (varistor), and more particularly to such a unit composed of a ceramically manufactured, monolithic body, having a plurality of layers of varistor material.
2. Prior Art
Voltage-dependent resistors, or varistors which are manufactured with multi-layer technology are described in "Advances In Ceramics" (American Ceram. Society, 1981) Vol. 1, pp. 349-358. The average grain size is specified as 10 .mu.m. The threshold voltage per grain boundary amounts to about 2-3 V. The specifications for the thickness of the varistor material layers are 20 .mu.m through 200 .mu.m, and properties are described which were measured with varistors having a layer thickness of 40 .mu.m or 150 .mu.m, composed of 20 layers stacked on top of each another. The non-linearity coefficient .alpha. is in the range of 20-30, and the varistor voltage, measured at one mA, is near the range of 4-40 volts.
Metal layers for contacting the coating are situated on the surface of the monolithic body and are formed of stoved silver. The publication referred to above does not provide details concerning the coatings within the interior of the monolithic body, nor the porosity of the material which is used.
The publication "Journal of Applied Physics", No. 54, May 5, 1983, pp. 2764-2772, describes low voltage varistors. In a reference to the American Ceramic Society publication referred to above, it describes that varistors made with multi-layer technology exhibit reduced current density at over-voltages, high capacitance without a resolution of the fundamental problem of the grain size distribution.
The Siemens brochure "Edelgasgefullte Uberspannungsableiter; Metalloxid-Varistoren SIOV", Nov. 4, 1984, pp. 44-63, describes the theoretical bases of metal oxide varistors using zinc oxide, and gives standard definitions of the applicable terms. Thus, the non-linearity coefficient .alpha. is specified on page 48 of the Siemens brochure as ##EQU1## in which I.sub.2 is one ampere, I.sub.1 =1 mA, U.sub.2 is the voltage measured at 1 A, and U.sub.1 is the voltage measured at 1 mA. The voltage that is measured at 1 mA is defined as the "varistor voltage", and is used for the classification of varistors.
Low voltage varistors which are manufactured according to standard technology have grain sizes of about 10 .mu.m and larger, in order to keep the number of grain boundaries between the coatings low. The use of such a coarse material, however, leads to the problem that the grain size distribution scatters greatly and thus the steepness of the voltage-current characteristic (the non-linearity coefficient .alpha.) drops greatly. Low-voltage varistors manufactured in this way are usually not suitable for protection against higher voltages because the units cannot dissipate the heat adequately which arises in the ceramic body.